High-intensity focused ultrasound (HIFU) may be used to perform non-invasive thermal ablations. Conventional HIFU therapies rely on classical absorption of ultrasound energy through viscosity and heat conductivity losses that induce temperature increase in tissues. Usually the thermal ablations offer a good predictability both in temperature increase and ablated area. In volumetric HIFU therapy the focal point is steered electronically along concentric circles in the targeted area in order to achieve a controlled temperature increase on a larger area and hence, a larger ablated area.
If the target area is deep seated or for example well perfused, the applied energy needs to be increased either by increasing the acoustic intensity or the duration of the sonication. However, this increases also the thermal dose received by the intervening tissues and in some cases the heating of the skin might become critical and prevent the therapy.
When tissues are exposed to high-intensity ultrasound waves there is a risk of cavitation or other bubble activity. Usually the cavitation is tried to be avoided during HIFU therapy due to its unpredictable nature.
United States Patent Application Publication US 2010/0241036 discloses a system in which a first level of ultrasound is delivered to a target tissue resulting in the generation of micro-bubbles and delivers a second level of ultrasound energy to the target tissue region for a second duration.
International patent application publication WO 2003/097162 describes a focused ultrasound system which uses micro bubbles in a tissue region to enhance tissue coagulation.